1. Field of the Invention
The present invention generally relates to a high voltage power supplying apparatus using a piezoelectric transformer, and particularly relates to an image forming apparatus using the high voltage power supplying apparatus.
2. Description of the Related Art
An electrophotographic image forming apparatus needs a high voltage power supplying apparatus to generate a voltage (high voltage) higher than several hundred V. This is because the image forming apparatus applies a DC bias voltage to a transfer member. Although a conventional high voltage power supplying apparatus uses a wire-wound electromagnetic transformer, a recent high voltage power supplying apparatus uses a piezoelectric transformer (piezoelectric ceramic transformer).
Japanese Patent Laid-Open No. 11-206113 discloses a high voltage power supplying apparatus having a voltage-controlled oscillator (VCO) that generates a frequency to be input to a piezoelectric transformer. The piezoelectric transformer has a characteristic that an output voltage is maximum at a resonance frequency. The high voltage power supplying apparatus described in Japanese Patent Laid-Open No. 11-206113 controls the output voltage of the piezoelectric transformer by controlling a frequency output from the VCO.
In general, to increase the output voltage of the piezoelectric transformer, the VCO, for example, changes the frequency of the piezoelectric transformer from a high frequency to a low frequency (or vice versa). The VCO controls the rising time and amount of the output voltage by controlling the change amount of the frequency. Therefore, as a desired voltage becomes higher, the change amount of the frequency becomes larger, thereby prolonging the rising time. In the high voltage power supplying apparatus of an image forming apparatus, for example, as the rising time becomes long, a time taken to start up the apparatus also becomes long. This may prolong a time necessary for preparing for image formation.